Your business code is a six-digit number that represents the kind of work you do. Different kinds of work have different business codes.
To enter the code, you will click on Edit for General Information in your Schedule C.

Then Edit in Tax details
Select Lookup Business Codes, enter some keywords that describe your business (for example, day care, bookkeeping, or door-to-door), and then select Search.
Select the code that most closely resembles your business's activities. If you can't find a code that applies, you can enter 999999.
If you need extra time and file an extension, it will waive failure-to-file penalties until October 18th. However, if you would have owed tax on April the 18th, the IRS will assess monthly late payment penalties and interest until you pay.
I recommend to extend your return with a TurboTax Extension and also pay your best guess of anything you may owe. If you overpay, it will be refunded. If you underpay, it will only calculate penalties on the part that was not paid.
TurboTax has a tool called TaxCaster to give you an idea of your tax liability and/or refund. You can vary different items and see how that could impact your return. Click for TaxCaster
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